The wrist includes seven carpal bones in two rows. The proximal carpal row consists of the triquetrum, lunate and scaphoid bones. The distal carpal row consists of the hamate, capitate, trapezoid and trapezium bones. An eighth carpal bone, the pisiform, rests anterior to the triquetrum. Collectively, the carpal bones are known as the carpus. The bones of the hand include five metacarpal bones that articulate at their proximal ends with the distal row of carpal bones. The forearm has two bones, the radius and ulna. The distal end of the radius articulates with the scaphoid and lunate bones and the distal head of the ulna. The joints between the carpal bones themselves and between the carpus and the radius permit wrist and hand movement. The carpus is supported by ligaments some of which run from various carpal bones to the distal ends of the radius and the ulna. In normal wrists, the approximate center of rotation for radial-ulnar deviation and flexion-extension motion is located at a point in the capitate bone known as the head of the capitate. This center of rotation is offset towards the palm and the ulnar side of the long axis of the radius.
Total wrist endoprosthetic arthroplasty involves the prosthetic replacement of the wrist by an artificial joint designed to simulate normal wrist motion while maintaining stability of the components and proper hand on forearm alignment. To achieve a stable full range of motion, one must reproduce as best as possible the anatomical situation. Where possible, ligaments should not be sacrificed. The prosthesis should place the ligaments in their proper degree of tension. The articulating surfaces should approximate the center of rotation and distribute forces over a wide area. This would provide some inherent stability while allowing the ligaments to limit the range of motion. In addition, the prosthesis should avoid invading non-diseased joints.
Prior devices are available for either total or partial replacement of the wrist joint. These devices employ various means of articulation including ball and socket joints. Orthopedically suitable materials are available including biologically inactive metals and plastics. One such plastic is high density polyethylene which is particularly suitable for concave shaped bearing surfaces in the articulation means of a prosthesis. Various means of attachment of a prosthesis to bone are available including pins, screws, intramedullary stems, and bone cement.
Two early prosthetic devices having variations of a ball and socket articulating means (Meuli; and Volz) are described by Cooney, W. P. et al "Total Wrist Arthroplasty; Problems with Implant Failures"; Clin. Orth. Rel. Research 187; 121-128 (1984) wherein it was reported that with such implants, there is an unacceptably high failure rate and a survival rate which decreases over time.
A variety of prosthetic devices are described in the following group of references:
Swanson, A. B. et al; "Flexible Implant Arthroplasty of the Radiocarpal Joint:, Clinical Orthopaedics and Related Research (1984), No. 187, p. 94-105. PA1 U.S. Pat. No. 4,040,130 (Laure) PA1 U.S. Pat. No. 4,063,314 (Loda) PA1 U.S. Pat. No. 4,100,626 (White) PA1 U.S. Pat. No. 4,178,640 (Buechler, et al) PA1 U.S. Pat. No. 4,180,871 (Hamas) PA1 U.S. Pat. No. 4,229,841 (Youm, et al) PA1 U.S. Pat. No. 4,229,840 (Gristina) PA1 U.S. Pat. No. 4,307,473 (Weber) PA1 U.S. Pat. No. 4,259,752 (Taleisnik) PA1 U.S. Pat. No. 4,645,505 (Swanson) PA1 U.S. Pat. No. 4,714,476 (Ranawat, et al) PA1 U.S. Pat. No. 4,784,661 (Beckenbaugh, et al) PA1 U.S. Pat. No. 5,133,762 (Branemark) PA1 European Patent Application No. 0 034 192 (Bedeschi and Luppino) PA1 Canadian Patent No. 1,053,852 (Frey) PA1 means for proximal fixation of the prosthesis that obviates the need for an offset in order to provide a center of rotation of the wrist near the anatomical center of rotation (the region of the head of the capitate) and provides proximal support for the prosthesis centered beneath the center of rotation for better support of the wrist; PA1 proximal fixation means for the prosthesis that allow one to avoid loss of the ligamenture between the carpus and the radius and ulna; PA1 distal fixation of the prosthesis which will provide better support for the wrist without fixation to a metacarpal bone thereby avoiding invasion of a carpal-metacarpal joint; and PA1 articulation means for a prosthesis comprising complementary bearing surfaces of sufficient size and appropriate shape to provide adequate support for the wrist while facilitating radial-ulnar deviation. PA1 (a) a proximal part which includes a plate configured to be supported on the distal surfaces of each of the radius and ulna when said radius and ulna are fused at their distal ends; PA1 (b) a distal part configured to be supported by surfaces on the distal carpal bones and having a proximal side defining an ellipsoidal convex bearing surface having a generally part-circular cross-section; and, PA1 (c) an intermediate component configured to be attached to said proximal part, and having a receptacle portion distally defining a concave bearing surface that is complementary to the convex bearing surface of said distal part; wherein the said distal part and intermediate component replace the proximal row of carpal bones and the said convex bearing surface is received in the concave bearing surface when the prosthesis is implanted in a wrist.
With the exception of Branemark, all the aforementioned prior art devices employ proximal fixation into only the radius. In the case of the devices of Laure and White, and that described in the reference by Swanson, et al, the center of rotation is in a nonanatomical position in line with the axis of the radius. The devices described in the other references and the aforementioned Meuli device provide articulating means offset from the axis of the radius placing the center of rotation in a more anatomical position. However, in the devices that have proximal fixation to only the radius, proximal support of the prosthesis is not centered underneath the center of rotation of the wrist.
The devices described in the aforementioned references (with the exception of the Swanson patent) all employ metacarpal fixation of the distal portion of the prosthesis. In the Beckenbaugh, et al patent, it is stated that the capitate-third metacarpal fixed unit is the logical positioning choice for the metacarpal component of the wrist prosthesis. In the Swanson patent, there is no distal fixation means because the distal portion of the device is a cup-shaped recess dimensioned to receive and articulate with a portion of the proximal row of the carpal bones. Thus in all cases where the prosthesis is fixed at its distal end, there is an invasion of one or more joints between the distal carpal bones and the metacarpal which, in many cases, are not diseased.
There is no mechanical joint which functions as an articulating means in the device of the Swanson patent. The device described in the reference by Swanson, et al is a single piece flexible silicone implant. Ball and socket joints are employed as articulating means in the devices described by Hamas, White, Frey, Laure, Loda, and Gristina. The aforementioned Volz device and the device described in the Weber patent have variations of ball and socket joints with sockets having part-cylindrical bearing surfaces. Youm, et al describes a joint arranged about pivot pins set on perpendicular axes. Buechler, et al describes a toroidal base that rotates in a part concave surface between two shafts. Ranawat, et al describes an articulating means employing a metal axle.
The device described by Taleisnik has proximal and distal components having elongated stems for intramedullary implantation through the capitate into the third metacarpal and into the radius. The proximal and distal components are connected by a variation of a ball and socket joint offset toward the ulna and the palm. The joint comprises a part-cylindrical recess engaged with a component having a circular cross-section and an oval shape in a longitudinal direction. At least the lunate bone is resected in order to permit implantation of the device. Proximal support is not centered underneath the center of rotation and the distal component invades the joint between the capitate and metacarpal.
Bedeschi and Luppino's European patent application describes a prosthesis having a plate-like first element which is fixed to the end of the radius and having a doubly concave bearing surface. The prosthesis includes a second element having a banana-like shape complementary to the bearing surface of the first element and a geometry that permits it to replace the scaphoid and lunate bones. The second element has fixing pins which are driven through distal carpal bones into metacarpal bones. The device does not provide an offset center of rotation.
The device described in the Beckenbaugh, et al patent is compared to the aforementioned designs of Meuli and Volz in Beckenbaugh, R. D.; Total Wrist Arthroplasty; Review of Current Concepts; in "The Wrist and its Disorders" (1988), ed. by D. M. P. Lichtman; W.B. Saunders Company; Chapter 29, p. 439-445. The device of Beckenbaugh, et al comprises first and second components each having fixation stems for attachment to the third metacarpal and the radius. The articulating means is a joint consisting of an ellipsoidal, convex bearing surface and a complementary, concave bearing surface on the other component. The ellipsoidal surfaces have generally part-circular cross-sections. The prosthesis invades a carpal-metacarpal joint and is only attached to the radius. The center of rotation of the articulating surfaces is offset towards the palm and the ulna from the axes of the radial and metacarpal fixation stems. This device provides a large bearing surface in an appropriate anatomical position but the distal end of the radius must be resected in order to implant the device resulting in loss of ligamenture supporting the wrist. Furthermore, the ends of the elliptically profiled convex bearing surface limits the range of radial-ulnar deviation which is normally the function of the ligamenture.
The device described in the Branemark patent is proximally attached to the radius and the ulna and distally attached to the third metacarpal. The primary articulating means is an elastomeric material between the means for proximal and distal attachment. The distal ends of both the radius and ulna are resected and prepared to provide flat surfaces, with the prepared ulna being substantially shorter than the radius. The means for proximal attachment includes a plate having a stepped outer or proximal surface that is intended to engage with the prepared surfaces of the radius and ulna. Thus, a first, thin portion of the plate has a flat proximal surface that is supported on the prepared surface of the radius, and a second, thick portion of the plate has a flat surface offset from the first portion, to be supported on the prepared surface of the ulna. The thick portion of the plate houses a secondary articulating means which is a swivelling joint having an axial member that is part of the ulnar attachment means. The radius is permitted to rotate relative to the ulna for supination and pronation by the presence of the swivel joint, but this necessitates the positioning of the ulna at a distance from the radius to prevent the two bones from striking. Since the ulna must be considerably shortened to accommodate the swivel joint mechanism, all of the ligaments attached to the distal end of the ulna must be sacrificed. Furthermore, the Branemark device would be subject to stress that would eventually loosen the ulnar and radial attachment of the joint since supination and pronation causes the relative length of the radius to the ulna to change exerting leverage against the fastening devices.